C2‐domain containing calcium sensors in neuroendocrine secretion

Abstract

The molecular mechanisms for calcium-triggered membrane fusion have long been sought for, and detailed models now exist that account for at least some of the functions of the many proteins involved in the process. Key players in the fusion reaction are a group of proteins that, upon binding to calcium, trigger the merger of cargo-filled vesicles with the plasma membrane. Low-affinity, fast-kinetics calcium sensors of the synaptotagmin family -especially synaptotagmin-1 and synaptotagmin-2 -are the main calcium sensors for fast exocytosis triggering in many cell types. Their functions extend beyond fusion triggering itself, having been implicated in the calcium-dependent vesicle recruitment during activity, docking of vesicles to the plasma membrane and priming, and even in post-fusion steps, such as fusion pore expansion and endocytosis. Furthermore, synaptotagmin diversity imparts distinct properties to the release process itself. Other calcium-sensing proteins such as Munc13s and protein kinase C play important, but more indirect roles in calcium-triggered exocytosis. Because of their higher affinity, but intrinsic slower kinetics, they operate on longer temporal and spatial scales to organize assembly of the release machinery. Finally, the high-affinity synaptotagmin-7 and Doc2 (Double C2-domain) proteins are able to trigger membrane fusion in vitro, but cellular measurements in different systems show that they may participate in either fusion or vesicle priming. Here, we summarize the properties and possible interplay of (some of) the major C2-domain containing calcium sensors in calcium-triggered exocytosis. Keywords: calcium sensor, exocytosis, Munc13, neuroendocrine, synaptic transmission, synaptotagmin. This article is part of a mini review series: "Synaptic Function and Dysfunction in Brain Diseases". Cellular processes as distinct as neurotransmitter release, mast cell degranulation, and hormone release are brought about by calcium-dependent exocytosis (Lindau and Gomperts 1991;Bean et al. 1994;Sudhof 2013). Extensive research over several decades has elucidated the general mechanism behind those processes, and the functions of the key players involved. It is now clearly established that, both in synaptic and endocrine secretion, the vesicle fusion machinery is composed, at its core, by a set of proteins forming the soluble N-ethylmaleimide sensitive factor attachment receptor (SNARE) complex. The canonical (neuronal) SNARE complex consists of the vesicular SNARE protein synaptobrevin 2/VAMP-2 and the plasma membrane SNAREs synaptosomal-associated protein of 25 kDa (SNAP-25) and syntaxin-1 (Sollner et al. 1993). The~65-residue heptad-repeat SNARE motifs of these proteins form a parallel four-stranded helix bundle (Hanson et al. 1997;Sutton et al. 1998) that can zipper-up from its Nto C-terminus (Hua and Charlton 1999;Sorensen et al. 2006) in a stepwise manner (Gao et al. 2012;Min et al. 2013) to bring the two opposing membranes together. The formation of this trans alpha-helical ternary complex...

Regulation of insulin exocytosis by calcium-dependent protein kinase C in beta cells

“…As reserve vesicles translocate to the plasma membrane, PKC may help to determine which vesicles successfully dock. Different isoforms of both munc18 and synaptotagmin display distinct calcium affinities that could be used to modulate exocytotic calcium sensitivity [124, 125]. Such a model is consistent with observations that PKC acts to increase the size of the highly calcium-sensitive vesicle pool [156–158].…”

“…Synaptotagmin is the primary calcium sensor of exocytosis, making it an ideal candidate for PKC-mediated calcium sensitization of the exocytotic apparatus [125] (Figure 1B3). However, phosphorylation of synaptotagmin does not appear to directly affect its calcium affinity.…”

“…The protein is phosphorylated by PKC at Thr112, and phosphorylation levels increase when PKC is activated by PMA [126]. There are many synaptotagmin isoforms, with Syt-1 and Syt-7 playing major roles in neuronal and endocrine secretion, respectively [125]. Synaptotagmin isoforms have different affinities for calcium, suggesting that they could mediate exocytosis of distinct vesicle pools in vivo [125].…”

“…There are many synaptotagmin isoforms, with Syt-1 and Syt-7 playing major roles in neuronal and endocrine secretion, respectively [125]. Synaptotagmin isoforms have different affinities for calcium, suggesting that they could mediate exocytosis of distinct vesicle pools in vivo [125]. Sorting of Syt-1 and Syt-7 onto distinct vesicle pools occurs in chromaffin cells and affects exocytotic behavior [127].…”

Ca2+ channel clustering with insulin-containing granules is disturbed in type 2 diabetes

“…This is consistent with insulin secretion measurements (43) and illustrates the importance of Ca 2+ microdomains for efficient exocytosis. However, both the modest global cytosolic Ca 2+ increase and the generation of diacylglycerol (DAG) in response to ACh will recruit Munc13 and related proteins such as Ca 2+ dependent activator protein for secretion (CAPS) and double-C2 domain (Doc2) to the plasma membrane (44) and thereby accelerate granule priming (45, 46). Given our data and findings from another study (22), it can be speculated that this increase in Munc13 availability also leads to enhanced L-type channel association with granules and that both mechanisms may contribute to the rescue of first-phase secretion by ACh in diabetic GK rat islets (47).…”